Garmin GPS doesn't work well in wooded mountain bike trails in new york? You don't say. Clearly the device must be garbage.
Run your iphone GPS on the same course and see the results it gives you. GPS isn't magic, there's a radio signal traveling from 12000 miles away. Obviously obstructions will mess with the reception strength. That's why your radio goes out under a bridge, except your radio station is located 2-3 orders of magnitude closer than a GPS satellite.
Now a dedicated GPS device like a garmin is going to do WAY better than the one embedded in your smartphone, because it doesn't have the additional noise of all the other components running in the background. A device with GLONASS will do even better than that. Your secondhand one-time anecdotal experience seems at odds with the THOUSANDS of strava users who seem to do just fine tracking their progress.
Well...Considering New York is one of the MOST highly covered areas in the United States and the MTB course was close the city and not highly wooded (there was never a loss of signal or course only a wide variance of tracking off by more than 200 ft at many places) I would expect the 510 to register far more accurately given its dual satellite capacity and supposed specialization for just this type of usage. It failed miserably and even the owner was embarrassed (he was a bike shop owner) when we looked closely at the results which he admitted he had not really paid much attention to as far as accuracy. Like most users he just assumed it was accurate because Garmin said so on the box and the website.
And I would imagine that very few people bother to verify the accuracy of their Garmin's and Strava info, 99 pct take whats given them and don't question it. See thewashingmachinepost blog about all this if you don't believe me. The GPS and whatever Russian crap-sats are being used are not accurate even close to what users hope for and some here seem to believe with such vehemence.
But hey its your money not mine..